Hot Bar Reflow Soldering Fundamentals Comprehensive Manufacturer of Metalworking Machinery A high quality Selective Soldering Technology Content 1. Hot Bar Reflow Soldering Introduction page 3-5 2. Application Range page 6-7 3. Process Descriptions page 8-13 > Flex to PCB > Wire to PCB 4. Design Guidelines page 14-22 5. Equipment page 23-25 6. Troubleshooting Guide page 26-27 All data, images and text described and illustrated in this document are subject to change. Amada Miyachi Europe reserves the right to modify the specifications, the design and the illustrations at any time. © All rights reserved – September 2014 What is Hot Bar Reflow Soldering? HBR Introduction Pulsed heat Thermode (Hot Bar) soldering, is a joining technology where two pre-tinned THERMODE HOLDER parts are heated to the melting point of the tin. The joining technology results in a HEAT IS THERMODE CONDUCTED permanent electro mechanical joint. FROM THE THERMODE TO THE PARTS AND The required process energy is supplied by a SOLDER thermode, also know as a Hot Bar. This PCB thermode is pressed on the upper part to transfer the thermal energy to both parts. Closed loop process control is used to control the time-temperature profile . How does it work? HBR Introduction Load PCB in Apply non-clean flux Load and position Start soldering customized fixture on pads Flex on the PCB process After a preset time Uniflow heats up the The reflow temp is Thermode moves uniflow ramps up to thermode to preheat kept on temperature down on the the reflow temperature to for a preset time soldering area temperature activate flux Thermode cools thermode moves up Hot Bar soldering down to the cool from the soldering process is temperature area completed The benefits of Hot Bar Reflow Soldering HBR Introduction • Suitable for mass production • Reliable processing, always equal process conditions • Cost effective due to the fact that no third component is needed to connect flex/wire to the PCB/substrate (connector or ACA can be avoided) • Multiple connections to be made simultaneously. Number of leads depend on product, pitch, design. • Fast temperature ramp-up and cool-down • Closed loop temperature and process control. • Very accurate positioning of the parts Hot Bar Reflow Soldering Applications Application Range Flex to Ceramic Component to PCB Flex to PCB Wire to PCB Hot Bar Reflow Soldering Applications Application Range Flex to PCB Flex to PCB Wire to PCB Leadframe to PCB Process Description: Preparation Process Descriptions In preparation for the Hot Bar Reflow soldering process, the following preparation steps need to be taken: 1. The base substrate is located in a fixture, and flux is applied to the pads. 2. The flex is positioned in the parts fixture, ensuring alignment of both sets of pads. 3. A process start signal is given to the soldering control unit. More info about the parts, the fixtures and the fluxing can be found further in this article. The Hot Bar Reflow Soldering process itself consists of the following process steps: heating up, reflow and cooling down. These process steps are described in the next sheets. Process Description: Contacting Process Descriptions The Hot Bar or thermode is mounted to a bonding head by means of a quick connect block. The bonding head has an accurate and stable linear guidance for the thermode. Movement is done with a pneumatic cylinder or an electrical motor. An internal spring system generated an accurate force. Most reflow joints of this nature require fewer than 100 Newton pressure. Force should be calibrated and set to the correct level to achieve the right transfer of thermal energy to the solder joint. The bonding head should have an accurate coplanarity adjustment to set the flatness of the thermode to the product accurately. These heads are modular in construction and therefore versatile for integration. After the start signal is given, the thermode is gently lowered until it seats on the product. The head senses this. Force is build up until the preset force is reached. When the right force is reached, a signal is passed to the power supply, which starts heating up the Hot Bar. Process Description: Heating up Process Descriptions By now, the Hot Bar holds down the product with the preset force. The Hot Bar is at “room temperature”. The solder control unit, also called “SCU” or “power supply” has received the s start signal for the soldering process. The SCU sends current through the Hot Bar. The Hot Bar is designed so that the electrical resistance is highest at the bottom (where it touches the product). Heat is generated because of the combination of current and electrical resistance. A small thermocouple is welded on the front of thermocouple. This thermocouple feeds back the actual Hot Bar temperature to the SCU. This makes a complete closed-loop regulation for the temperature-time cycle. Normal rise time for most thermodes is 1.5 to 2 seconds, equaling a heating rate of about 200 degrees Celsius a second. The newest generation of solder control unit is controlling the temperature all the way through the heating up phase. When the “REFLOW temperature” is nearly reached the solder control unit needs to slow down the heating rate to prevent a temperature overshoot. A good solder control unit an A good solder control unit and Hot Bar combination will compensate for all differences in heat-loads that can occur during normal production circumstances. Process Description: Reflow Process Descriptions During the reflow period the flux is activated, the flux cleans the surf aces and the solder is heated until it starts melting on all pads. This normally takes 3-8 seconds, at Hot Bar temperatures around 300°C (Hot Bar touches the leads), 400°C (Hot Bar touches kapton) or 500°C (ceramics and MC-PCB soldering). Although normal solder will melt at 180°C, ideal solder temperature is above 220°C to get a good flowing and wetting behavior but below 280°C to prevent burning of the solder. The Hot Bar must be set higher due to the thermal transfer losses. Ideally, time can be programmed on the SCU in 0.1- sec. increments and temperature in one-degree increments. Use the minimum time and temperature to achieve the desired joint to minimize the parts exposure to heat and chance of damage. Process Description: Cooling Process Descriptions When the solder is connected on all pads, the energy delivery to the Hot Bar can be stopped. The Hot Bar will start cooling down. The cooling process can be shortened by the use of forced air-cooling. The SCU can switch a relay that controls the flow of air at the end of the reflow period and cool the joint and Hot Bar rapidly. For optimum process control, cooling is done to a specific temperature. This temperature is set below the solder solidification temperature. Therefore, as soon as the solder becomes solid, the process is ended and a joint is formed. Because most connections have a relatively high heat sink, the temperature in the solder is lower than the measured Hot Bar temperature, even when using forced air-cooling. Therefore, the release temperature can be set to 180ºC in most cases without the chance of releasing the parts before solidification has taken place. Process Description: Quality Control Process Descriptions Quality control before starting the process is done by: The Amada Miyachi Europe force measurement kit, which contains a precision load-cell with a large contact area to support the Thermo-plane Thermode and a control panel including a readout display. The Amada Miyachi Europe pressure sensitive paper, especially designed to optimize the planarity of the Thermo-plane Thermode in combination with the product. Putting it in between thermode and product support will result in an accurate readout of planarity. Knowing and understanding the temperature during all the different Hot Bar processes inside the materials is essential for optimal results. The easy-to-use handheld temperature meter can be used in combination with ultra flat thermocouples. The thermocouple can be sandwiched between the parts, and the actual process temperature can be measured inside the connection. The thermocouple is re-usable. Quality control during the entire process cycle The new Premium Hot Bar Monitoring system combines MIYACHI EAPRO Hot Bar know- how with MIYACHI PECO technical expertise all in one unit. The MG3 Hot Bar measures force, temperature, time and displacement, allowing for continuous control throughout the entire process cycle. The compact and flexible system features high quality Hot Bar connections and high throughput, all within an easily adjustable frame construction. Quality control after the bonding cycle Using the MG3 for Hot Bar, featuring full on screen (SPC) statistic process control. Design Guidelines Flex Design: Connection Type Open windowed flex design: This design has both sides of the polyimide material removed from the joint area but has support from the remaining polyimide material on the sides and also along the end of the traces. This design gives some strength to the assembly and is resilient to harsher handling. As the traces are exposed, the thermal transfer to the parts is good and excess solder has extra space to flow. Thermode sizing is critical as it must fit into the window and allow space for the molten solder to flow. This design behaves similar to the exposed lead design. Flex Design: Connection Type Design Guidelines Single sided flex design: This design has the polyimide removed on one side only. Heat is conducted from the Hot Bar through the solid polyimide surface to the exposed traces underneath. The polyimide conducts heat through the insulation to the exposed traces and pads on the PCB.